This invention relates to a process for removing ammonia from organic solutions of an oxime-metal chelate prior to treating the solution to remove the metal therefrom.
In copending applications Ser. No. 266,985 entitled "Selecting Stripping Process", Ser. No. 266,981 entitled "Process for Recovering Nickel Selectively" and Ser. No. 266,986 entitled "Selective Solvent Extraction Process for Copper", all filed June 28, 1972 there are described various processes for recovering nickel or copper from aqueous ammonical solutions. In these processes, an ore containing copper and/or nickel is contacted with an aqueous ammonical solution, generally ammonia-ammonium carbonate having a pH above 9, to form an aqueous solution of these metals. The copper and/or nickel contained in the aqueous metal-rich solution then is chelated with an oxime in a solvent extraction step. The chelate is stripped with an aqueous acid solution to transfer the metal from the oxime to the acid solution and the metal-rich stripping solutions are treated by electrowinning to recover pure nickel and copper.
In one process, separation of nickel and copper is achieved in the solvent extraction step. Copper is selectively extracted from the aqueous solution by effecting copper ion transfrom the aqueous solution to the oxime solution and hydrogen ion transfer from the oxime solution to the aqueous solution in accordance with the equation: EQU (2RH) org. + (Cu.sup.++)aqu. .fwdarw. (R.sub.2 Cu)org. + (2H.sup.+)aqu.
wherein R is the organic moiety of the oxime and wherein the mole ratio of exchangeable hydrogen ion in the oxime solution prior to contact with the aqueous solution to copper ion in the aqueous solution prior to contact with the oxime solution is between about 1.8 and 2.4. Thereafter, a copper-barren, nickel-rich aqueous solution recovered from the copper extraction step is contacted with a second organic oxime solution in a second extraction step to effect nickel ion transfer from the aqueous solution to the oxime solution and hydrogen ion transfer from the oxime solution to the aqueous solution. The copper-rich oxime solution and the nickel-rich oxime solution obtained from the extraction steps are treated in separate stripping steps with an acid aqueous solution followed by electrowinning to recover nickel metal and copper metal separately.
In an alternative process, selective separation of nickel and copper is effected in a stripping step following coextraction of nickel and copper from an aqueous ammonical solution with an organic oxime solution. In this process, hydrogen ion in the aqueous solution is exchanged for nickel ion in the oxime selectively. Nickel and hydrogen ion transfer is effected in the nickel stripping step in accordance with the following equation: EQU (R.sub.2 Ni)org. +2(H.sup.+)aqu. .fwdarw.(2RH)org. + (Ni.sup.+.sup.+) aqu.
wherein the mole ratio of hydrogen ion in the fresh aqueous acid solution to nickel ion in the fresh oxime solution is maintained between about 1.8 and 2.2. A copper-rich oxime solution recovered from this acid nickel stripping step 15 contacted with an aqueous acid solution in a separate copper stripping step wherein the copper ion in the oxime solution and hydrogen ion in the aqueous acid solution are exchanged. The nickel-rich aqueous solution and the copper-rich aqueous rich solution recovered from each of these stripping steps then are treated by electrowinning in separate steps to obtain pure nickel and copper metal separately.
It has been discovered that in each of these processes, some ammonia is transferred from the original ammonical leach solution to the oxime during extraction and essentially all of this ammonia is transferred to the acid aqueous stripping solutions. This transfer results in substantial reagent losses and formation of ammonium salts which must be removed from the process. The formation of the ammonium salts in the stripping step is particularly undesirable in the case of nickel stripping since it results in the precipitation of nickel as NiSO.sub.4 .sup.. (NH.sub.4).sub.2 SO.sub.4 when the preferred sulfuric acid stripping solution is employed. Accordingly, to render the processes described in the above identified applications attractive, from a commercial standpoint, it is necessary that the ammonia be removed from the organic oxime solution prior to contact with an aqueous stripping stream in the stripping steps.
Since ammonia is soluble in water and in a wide variety of aqueous salt solutions, it would be expected that ammonia could be scrubbed from the oxime-metal chelate organic solutions by countercurrent contact therewith in a plurality of stages. However, it has been found that scrubbing of ammonia from saturated organic solutions with water and various aqueous salt solutions is impractical because of a low distribution ratio of ammonia between the aqueous and organic phase. Also, when water is contacted with the ammonia containing, oxime-metal chelate, organic solution, an emulsion is often formed which hinders or prevents separation of the aqueous phase from the organic solution thereby causing ammonia to be retained in the organic solution. It is believed that the ammonia forms ammonium hydroxide with the water and that the ammonium hydroxide promotes emulsion formation. Thus, the use of water would require means for breaking the emulsion prior to directing the organic solution to a stripping step.
As used herein, the terms, "first scrubbing stage" or "first stage" refers to the stage in the multi-stage scrubbing process wherein ammonia-rich organic solution first is contacted with an aqueous solution containing carbonate or bicarbonate ion. The terms "last scrubbing stage" or "last stage" refers to the stage in a multi-stage scrubbing process wherein fresh aqueous solution containing carbonate or bicarbonate ion is contacted first with the ammonia-rich organic solution and wherein ammonia-depleted organic solution is removed from the scrubbing process. The remaining stage in the scrubbing process are referred to herein as the "intermediate stages".